Oscillators employing MEMS (Micro Electro Mechanical Systems) technology, in which vibrating portions including a piezoelectric thin film are configured upon a silicon semiconductor layer, have been under development for some time. In oscillators, there is demand minimal variation in oscillation frequency even if an ambient temperature varies, and a technique such as that described hereinafter has been disclosed as an oscillator using a MEMS structure.
For example, an oscillator has been disclosed in which a heater constituted of a resistor and a temperature sensor are formed upon a Lamb wave-mode MEMS device in which a piezoelectric thin film is formed on a silicon substrate, and the oscillator is driven by electrodes formed above and below the piezoelectric thin film (see Patent Document 1, for example). This oscillator is held by an I-shaped holding portion having an overall rectangular plate shape with portions extending from the centers of two opposing sides thereof. The oscillator is used as a resonator that resonates at a frequency based on the rectangular plate shape in response to a voltage being applied to the piezoelectric thin film of the MEMS device. The temperature sensor, which is formed in the vicinity of a rectangular plate-shaped vibrating structure, measures the temperature of the vibrating structure itself, and the heater is controlled so as to maintain a constant temperature. This provides a highly-precise temperature-compensated oscillator with an oscillation frequency with minimal variation even when the ambient temperature varies.
An oscillator employing a contour vibration (breathing vibration) mode electrostatic MEMS device that includes two T-shaped holding portions extending from central areas on two opposing sides of a rectangular plate has been disclosed as another example (see Patent Document 2, for example). This document discloses a MEMS device in which two resistive films are formed integrally on a line that connects the holding portions. This MEMS device vibrates in a contour mode in response to an electrostatic force between an electrode formed on a vibrating body and an electrode formed on an anchoring portion. According to this oscillator, an oscillation frequency is adjusted, a heater is controlled to maintain a set temperature, and so on by using the resistive film to measure a temperature of a resonator and controlling a DC bias so as to correct skew between a frequency at that temperature and a reference frequency. This design provide a highly-precise temperature-compensated oscillator.
Patent Document 1: U.S. Pat. No. 8,410,868.
Patent Document 2: European Patent Application Publication No. 2,530,836.
Incidentally, with the oscillator according to Patent Document 1, in the Lamb wave mode, an amplitude of vibrations is high in peripheral portions of the rectangular plate and a thin-film resistance deforms by contracting in response to vibrations. Accordingly, variations in resistance caused by vibrations are superimposed as noise over variations in resistance caused by temperature, and a highly-accurate temperature measurement cannot be taken. As such, a highly-precise oscillator cannot be realized. Additionally, the deforming contractions are large, leading to issues in terms of reliability such as peeling or the like.
On the other hand, when an electrostatic force is applied to elicit expansions and contractions in a direction perpendicular to the direction of the straight line connecting the holding portions as in the oscillator according to Patent Document 2, there is a high degree of coupling in the straight line direction when the vibrating mode is the contour vibration mode. This produces small, but noticeable expansions and contractions in the straight line direction along the straight line that connects the holding portions as well (that is, the direction in which the holding portion extends). As such, according to this oscillator, a resistance value of the resistive film will vary in response to the expansions and contractions in the straight line direction, and a highly-accurate temperature measurement cannot be taken. Thus, highly-accurate temperature correction cannot be provided. Note that in Patent Document 2, the holding portions are formed having T shapes in order to reduce expansions and contractions along the straight line connecting the holding portions.